Recently, interest in safety has been increasing while performance and power of an automobile have been improved. A tire is not an exception, and particularly in a tread portion, demands for improving acceleration characteristic and grip performance represented by braking performance have been increasing. For example, improvement in various performances at high speed running is one of them.
Grip performance is dependent on hysteresis loss characteristic of a rubber composition, and for improving the grip performance of a rubber composition, for example, a method of increasing a styrene content and vinyl bond amount of a styrene-butadiene rubber (SBR) to further raise a glass transition temperature (Tg) has been known. However, in this case, not only abrasion resistance is lowered, but also the grip performance at low temperature is lowered, and there is also danger of causing brittleness fracture. Further, for improving the grip performance, a method of using much amount of oil has also been known, but the abrasion resistance is lowered due to lowering of fracture characteristic.
Additionally, a method of using a low molecular weight SBR is also proposed as a method of improving grip performance. However, balance between the grip performance and the abrasion resistance is not adequate. There are also proposed a method of adding an inorganic compound such as tungsten to a rubber composition (for example, refer to JP2000-319447A), a method of adding an acrylic resin (for example, refer to JP2002-80642A), and a method of adding urethane particles (for example, refer to JP2002-97303A). However, a rubber composition exhibiting adequate grip performance has not been obtained in all the cases.